A conventional channel estimation technology will hereinafter be described in detail.
A process of estimating and compensating for a signal distortion which may occur due to multipath fading, etc. is called “channel estimation”. This channel estimation is roughly classified into a pilot signal-based scheme and a data-based scheme according to the format of data used for the channel estimation. The pilot signal-based scheme is adapted to allocate a part of a time or frequency domain to a specific signal for the channel estimation.
FIG. 1 illustrates a conventional method for configuring a pilot signal in a communication system (for example, an OFDM/OFDMA, FDM/FDMA, TDM/TDMA or CDM/CDMA communication system).
In a conventional wireless communication system, data is allocated to a specific subcarrier, specific frequency band, specific time slot or specific code. A pilot is also allocated to a frequency-time resource other than the allocated subcarrier, frequency band, time slot or code.
The pilot means a pilot signal. As mentioned above, the OFDM/OFDMA, FDM/FDMA, TDM/TDMA and CDM/CDMA communication systems multiplex the data and pilots using the subcarriers, frequency bands, time slots and codes, respectively.
The data or pilot multiplexed using the subcarrier, frequency band, time slot or code is referred to as a “sample.” For example, the sample in the OFDM/OFDMA system represents a data signal or pilot signal transmitted at a specific subcarrier, the sample in the FDM/FDMA system represents a data signal or pilot signal transmitted at a specific frequency band, the sample in the TDM/TDMA system represents a data signal or pilot signal transmitted for a specific time slot, and the sample in the CDM/CDMA system represents a data signal or pilot signal transmitted through a specific code. These respective samples are transmitted through specific frequency-time resources (for example, a specific subcarrier, specific frequency band, specific time slot and specific code).
An index “m” will hereinafter be used for distinction of the frequency-time resources based on indexes. For example, the index “m” for a specific sample in the OFDM/OFDMA system is a data symbol index for distinction of a subcarrier on which the specific sample is transmitted.
Also, the index “m” for a specific sample in the FDM/FDMA system is a frequency index for distinction of a frequency band at which the specific sample is transmitted.
Also, the index “m” for a specific sample in the TDM/TDMA system is a time index for distinction of a time slot for which the specific sample is transmitted.
Also, the index “m” for a specific sample in the CDM/CDMA system is a code index for distinction of a code applied to the specific sample.
A description will hereinafter be given of a process of performing channel estimation using a pilot signal and decoding data according to the channel estimation.
At a receiving stage, data transmitted from a transmitting stage can be accurately restored by, through the following procedures, estimating a channel using a pilot and compensating for the value of the estimated channel.
Assuming that a transmitted signal is “d”, a channel is “h”, an additive white Gaussian noise (AWGN) is “v”, and a received signal is “x”, the received signal “x” can be expressed as in an equation x=h·d+v, and, at the receiving side, a channel ĥ can be estimated through this equation because a pilot value is known in advance.xm/ĥ=h·d/ĥ+v/ĥ≅d+ v  [Equation 1a]
Substituting the value of the estimated channel into the above equation 1a, the data d can be finally restored.
In the aforementioned channel estimation method, accuracy of the channel estimation value ĥ using the pilot is important. The channel of the pilot is not accurately equal to that of the data.
However, the closer the pilot-based channel estimation value is to a time or frequency axis, the higher the similarity between the pilot channel environment and the data channel environment, such that the above-mentioned pilot-based channel estimation information is used for the data recovery.
In other words, the closer the pilot is to the data, the higher the channel estimation throughput (or performance). The higher the number of pilots, the higher the channel estimation throughput, such that the data recovery is implemented. However, the allocation of a large number of pilots means that a large amount of resources to be allocated to the data are consumed, such that it is very important to properly arrange the above-mentioned pilots.
The OFDMA technology will hereinafter be described in detail.
Firstly, an OFDM (Orthogonal Frequency Division Multiplexing) technology will be described for the convenience of description. The OFDM technology converts input data into parallel data units equal to the number of used subcarriers, loads the parallel data units in each subcarrier, and transmits the subcarriers including the parallel data units, such that it is considered to be a MultiCarrier Transmission/Modulation (MCM) scheme employing a variety of subcarriers.
Secondly, the OFDMA (Orthogonal Frequency Division Multiple Access) technology will be described in detail. The OFDMA scheme allocates a different number of subcarriers according to a transfer rate requested by each user, such that it can effectively distribute resources. Similar to the OFDMA-TDMA scheme, the OFDMA scheme need not execute the initialization using a preamble before each user receives data, resulting in increased transmission efficiency.
Specifically, the OFDMA scheme is suitable for a specific case where a large number of subcarriers are used, such that it can be effectively used for the wireless communication system equipped with a broad-area cell having a relatively-high Time Delay Spread (TDS).
Also, a frequency-hopping OFDMA scheme solves the problems generated when a subcarrier-interference caused by other users or a deep-fading subcarrier occurs in a radio or wireless channel, such that it increases a frequency-diversity effect and acquires an interference-averaging effect.
FIG. 2 is a conceptual diagram illustrating a pilot allocation scheme for use in an OFDM-based wireless communication system.
The IEEE 802.16 system performs the pilot allocation using the pilot allocation scheme shown in FIG. 2. A pilot allocation scheme for the OFDM-based wireless communication system will hereinafter be described with reference to FIG. 2. The IEEE 802.16 system may have 128 subcarriers, 512 subcarriers, 1024 subcarriers, or 2048 subcarriers. Some parts of both sides of a total of subcarriers are used as a protection band. In the case of the remaining parts other than the above-mentioned parts, a single subcarrier from among 9 subcarriers is allocated to the pilot, and the remaining 8 subcarriers other than the single subcarrier may be allocated to data.
Conventional TDM/TDMA, FDM/FDMA, and CDM/CDMA technologies will hereinafter be described.
FIG. 3 is a conceptual diagram illustrating a conventional pilot allocation scheme for a TDM/TDMA-based wireless communication system.
Referring to FIG. 3, a data signal is assigned to each time slot according to the TDM/TDMA scheme. The data signal is contained in a first timeslot, and the pilot signal is contained in a second timeslot, such that the first and second timeslots including the data and pilot signals are transmitted to a destination.
FIG. 4 is a conceptual diagram illustrating a pilot allocation scheme for a CDM/CDMA-based wireless communication system.
Individual data signals are distinguished from each other by different codes according to the CDM/CDMA scheme. Preferably, in the case of multiplexing individual data units, the above-mentioned different codes may be orthogonal codes to allow a reception end to detect the data units while being classified.
As shown in FIG. 4, each data signal and each pilot signal are distinguished from each other by different codes, such that they are transmitted to a radio channel.
In the case of the pilot allocation based on the FDM/FDMA technology, individual data signals are classified according to a frequency band for transmission of the above-mentioned signals. Preferably, a predetermined protection area may be formed between the above-mentioned frequency bands to reduce interference between several data signals. In the case of the FDM/FDMA technology, the above-mentioned data signal and the pilot signal are distinguished from each other according to different frequency bands, such that they are transmitted to the radio channel.
It is assumed that the wireless communication system based on OFDM/OFDMA, FDM/FDMA, TDM/TDMA, or CDM/CDMA technology uses the aforementioned conventional pilot-signal usage method. Under this situation, the higher the number of the pilot signals, the higher the channel estimation throughput. However, the action of increasing pilot signals within limited frequency/time resources unavoidably encounters the reduction of data transmission resources.
For example, if the OFDM/OFDMA-based wireless communication system allocates a single subcarrier from among 9 subcarriers to the pilot, this means that radio resources for data transmission are reduced by the ratio of 1/9. Provided that a single subcarrier from among 3 subcarriers is allocated to the pilot to perform the channel estimation of a higher throughput, radio resources for data transmission are reduced by the ratio of 1/3.